Several DNA metabolic processes are influenced or controlled by genes whose products function in DNA repair. The RAD52 gene of Saccharomyces cerevisiae plays a central role in many of the events in recombination and repair. It controls the repair of ionizing radiation-induced DNA double-strand breaks, radiation-induced spontaneous mitotic recombination, and recombination during meiosis, and is required for mutagenesis by some alkylating agents. Utilizing an antibody raised against a Neurospora crassa deoxyribonuclease, we had shown that an antigenically-related enzyme could be identified from yeast and that this enzyme is controlled by the RAD52 gene. The protein has been purified and is a single-strand exo- endonuclease and a double-strand exonuclease with MW = 72,000. It is not the product of the RAD52 gene. Using a lambda gtll vector expression library that contains genomic yeast DNA and the antibody as a probe, we have identified a segment of DNA that codes for cross-reacting material. A sequence from a yeast genomic library has been identified that hybridizes with this segment. Introduction of this into a Rad+ strain leads to enhanced synthesis of cross-reacting material. The gene appears to be essential based on mutation of the cloned sequence and introduction into the genome. The transcription of this gene is under the control of the RAD52 gene.